1. Field of the Invention
This invention relates to a method of forming a colored oxide film on the surface of an aluminum material or aluminum alloy (hereinafter referred to merely as the aluminum material), and more particularly to a method with which it is possible that an oxide film of such color tone in the range from a bronze color to a silver color is formed on the surface of the aluminum material by anodizing the aluminum material in an aqueous solution of oxalic acid containing sulfuric acid.
2. Description of the Prior Art
Generally speaking, since the aluminum material is light and small in deformation resistance as compared with other metal materials, it is fit for various uses and often used as a construction material or the like. The surface of such aluminum material now on the market is reinforced by anodic oxidation. Recently, the aluminum material which is subjected to the surface treatment by anodic oxidation is often colored during the anodic oxidation treatment and put on the market as a colored aluminum material.
However, the aluminum material thus put on the market as a colored aluminum material still encounters various problems in the coloring method and the range of colors obtainable with known methods is narrow.
The known methods of coloring the aluminum material is (a) a method employing an aqueous solution of oxalic acid and (b) a method employing an aqueous solution of aromatic sulfonic acid. The method (a) using the aqueous solution of oxalic acid is one that the aluminum material is anodized in the aqueous solution of oxalic acid to form a yellow-colored oxide film on the surface of the aluminum material. This method is very excellent because the oxalic acid which is the fundamental liquid is easily available and inexpensive and because the conditions for electrolysis are easy from the industrial point of view. On the other hand, this method has such a defect that the range of tone color obtainable with this method is limited to yellow, in particular, to pale yellow.
The method (b) is one that the aluminum material is anodized in the aqueous solution of aromatic sulfonic acid, as is the case with the method (a), thereby forming a bronze-colored oxide film on the surface of the aluminum material. Unlike the method (a), this method (b) is capable of coloring the oxide film in bronze and the range of color tone obtainable with this method is very wide. However, the method (b) is defective in that the aromatic sulfonic acid, i.e. the fundamental liquid of the aqueous solution used, is extremely expensive and that a relatively large amount of such an expensive fundamental liquid is required.